It is very attractive from a viewpoint of a cost and a yield to monolithically integrate and circuitize a photodetector with a silicon electronic technology. For example, a silicon photoreceiver monolithically integrated and circuitized on a chip identical to that of a CMOS circuit (for example, a silicon photodiode) is one of attractive substitutes for a hybrid photoreceiver (for example, a InGaAs photodiode connected to the CMOS circuit or a GaAs circuit). The photoreceiver monolithically integrated and circuitized is manufactured through a standard silicon process. And, it is expected that the above photoreceiver is manufactured at a lower cost as compared with the hybrid-designed photoreceiver.
The photodiode is employed as a means for converting the optical signal into the electric signal at a high speed. A pin type photodiode is representative thereof. The pin type photodiode has a construction in which an i layer of an intrinsic semiconductor has been put between a p layer of a p-type semiconductor and an n layer of an n-type semiconductor. And, when an inverse bias voltage is applied to the pin type photodiode, almost all region of the i layer having a high resistance becomes a depletion layer of an electric charge carrier. A photon of incident light is mainly absorbed in the i layer. And electron/positive hole pairs are generated. Each of the generated electron and positive hole drifts within the depletion layer in an opposite direction to the other due to the inverse bias voltage. This allows a current to flow. And it is detected as a signal voltage with a load resistance. Main factors for governing a response speed of this optoelectric conversion are a circuit time constant and a carrier drift time. The circuit time constant is governed by a product of the load resistance and an electric capacity being produced by the depletion layer. The carrier drift time is a time necessary for the electron (positive hole) passing through the depletion layer.
There exists a Schottky type photodiode as a photodiode of which the carrier drift time is short. This photodiode is a photodiode having a construction in which a semitransparent metal film is in contact with the n layer (or n− layer) of the semiconductor. A Schottky junction is formed in the neighborhood of an interface in which the n layer (or n− layer) and the semitransparent metal film contact each other. Diffusion of the electron from the semitransparent metal film to the n layer (or n− layer) occurs in the neighborhood of this Schottky junction. And the depletion layer is formed. When the incident light is radiated in this state, the electron is generated in the n layer (or n− layer). And, the foregoing electrons drift within the depletion layer due to the inverse bias voltage. Further, the light absorption on the element surface layer can be effectively utilized. For example, the pin type photodiode necessitates the i layer (depletion layer) having a sufficient thickness because of absorption of the photon. However, the depletion layer of the Schottky type photodiode can be made thin. Thus, the carrier drift time can be shortened. Additionally, it has been proposed to adopt a lateral electrode structure, thereby to make a gap between the electrodes short so as to thin the depletion layer for the pin type photodiode as well (see Non-patent document 1). However, this technique, which enables the high speediness, is poor in a light absorption efficiency on the surface layer of the semiconductor. And, the high sensitivity is difficult to attain.
By the way, making a value of the load resistance small so as to make the circuit time constant short causes the voltage of a reproduction signal that is takable to lower. Thus, improving S/N of the reproduction signal (reducing an error in the reading-off) necessitates reducing an electric capacity of the depletion layer. In particular, making the depletion layer thin so as to make the carrier drift time short causes the electric capacity to be increased. Thus, it is necessary to reduce an area of the depletion layer (or the Schottky junction) so as to attain the high speediness. However, reducing the junction area causes a utilization efficiency of the signal light to lower. Resultantly, the S/N of the reproduction signal lowers.
In consideration of the above-mentioned problems, utilization of a metal surface plasmon (or a photonic crystal structure) has been proposed in an optoelectric conversion device. That is, an attempt for attaining the high speediness/miniaturization for the device is in progress.
For example, the technology of Patent document 1 has been proposed. A photodetector of the metal/semiconductor/metal (MSM) type in which two electrodes have been mounted on an identical surface of the semiconductor have been disclosed in this document. This MSM type photodetector is one kind of the photodiodes having the Schottky junction in the neighborhood of the two electrodes. One part of the light having transmitted through the electrode surface is absorbed in a semiconductor layer. And a photocarrier is generated. In this MSM type photodetector, making the semiconductor thick for a purpose of enhancing a quantum efficiency leads to an increase in a propagation distance of the photocarrier. As a result, an operational speed lowers. In the Patent document 1, so as to prevent this operational speed from lowering, it has been proposed to form the metal electrode along periodic roughness. That is, a scheme has been proposed for efficiently coupling the incident light to the surface plasmon of the metal electrode, and allowing it to propagate inside the photodetector.
Further, the technology of Patent document 2 has been proposed. The method has been disclosed of manufacturing an MSM type light receiving element in this document. That is, the method has been disclosed of forming the metal film on the semiconductor, partially oxidizing this metal film, and forming a light transmissive insulating pattern.
Further, the technology of Patent document 3 has been proposed. It has been disclosed in this document that the width of the light transmissive insulating pattern is made equal to or less than a wavelength, and proximity field light that occurs in an edge of the metal film existing in both sides of the light transmissive insulating pattern is utilized. Further, it has been disclosed that the response speed of this MSM type light receiving element is made fast.
Further, the technology of Patent document 4 has been proposed. In this document, an optoelectronic coupler has been disclosed in which a positive polarity and a negative polarity of the crossed finger type metal electrode, which are systematically spaced on the semiconductor, have been arranged in such a manner that they face each other as a nested function. Further, it is described that the incident light is coupled to each of the transmissive light, the reflection light, the surface plasmon, a polariton, etc. with the resonance. Further, it is also described that the optoelectronic coupler can be employed as an MSM type light receiving element. Further, it is also described that the photocarrier is intensified owing to the coupling of the incident light and the surface plasmon. However, in the case of employing the optoelectronic coupler described in the Patent document 4 as an MSM type light receiving element, reducing an irradiation area of the incident light for a purpose of reducing the electric capacity of the depletion layer leads to a decline in the intensity (S/N) of the detection signal. Further, there is no description of the coupling of the light energy being transmitted in the optical waveguide path and the semiconductor layer in the Patent document 4.
Further, the technology of Patent document 5 has been proposed. A photovoltaic device having periodically-arranged apertures (or concave portions) formed on one of two electrodes holding a plurality of spherical semiconductors each having a pn junction between them has been disclosed in this document. This photovoltaic device utilizes the resonance of the incident light and the surface plasmon in the electrode having the periodic shape. However, there is no description of making the depletion layer thin and yet making the area small for a purpose of attaining the high speediness of the optoelectric conversion in the Patent document 5.
Further, the technology of Patent document 6 has been proposed. An MSM light receiving element having the light absorption layer as a layer having a multilayer film structure so as to form a photonic band has been disclosed in this document. Further, it is described that making a group velocity of the light, which is absorbed and transmits, small allows the physical absorption layer to be made thin. Further, it is described that with this, the light receiving efficiency is enhanced. However, the point of reducing the junction area in the MSM junction and making the element capacity small has not been realized also in this structure.
Further, the technology of Patent document 7 has been proposed. An MSM type photodetector in which the metal electrode has been formed with the semiconductor absorption layer penetrated by it has been disclosed in this document.
Further, the technology of Patent document 8 has been proposed. An optical transmitter utilizing a metal film having the aperture and the surface shape being periodically changed has been disclosed in this document. However, the technology described in the Patent document 8 is not a technology associated with the optoelectric conversion device. Additionally, it is described that, notwithstanding a single aperture, forming an array of the periodic grooves around the above aperture makes it possible to intensify the light that propagates. That is, it is described that the light that propagates can be intensified as compared with the case of having no array of the periodic grooves. However, it is known that total energy of the light that transmits attenuates as compared with the energy of the incident light. For example, the Non-patent document 2 says that the total energy of the light that transmits through the aperture of which the diameter is equal to or less than 40% of a wavelength attenuates to 1% of the incident light energy or less. Thus, the high S/N is not gained even though the light receiving element is irradiated with the propagation light from the optical transmitter described in the Patent document 8.
Further, the technology of Patent document 9 has been proposed. A structure has been disclosed in which a specific wavelength is optically coupled in a selective manner by employing a diffraction grid causing localized resonance to occur between the optical waveguide paths has been disclosed in this document. However, this technology is a technology relating to the optical coupling between the optical waveguide paths. And, the effect of localizedly entrapping the coupled light energy deemed to be necessary for the light receiving element as well as the structure in which the diffraction grid is used both as an original function and an electrode have not been realized.
Further, the technology of Patent document 10 has been proposed. An optical waveguide path structure utilizing the surface plasmon resonance has been disclosed in this document. However, in this technology as well, the coupling structure of the optical waveguide path and the photodiode, and the propagation structure of the surface plasmon are not utilized.    Patent document 1: JP-P1984-108376A    Patent document 2: JP-P1996-204225A    Patent document 3: JP-P1996-204226A    Patent document 4: JP-P1998-509806A    Patent document 5: JP-P2002-76410A    Patent document 6: JP-P2005-150291A    Patent document 7: JP-P2003-520438A    Patent document 8: JP-P2000-171763A    Patent document 9: JP-P2003-504659A    Patent document 10: JP-P2004-109965A    Non-Patent document 1: S. J. Koester, G. Dehlinger, J. D. Schaub, J. O. Chu, Q. C. Quyang, and A. Grill, “Germanium-on-Insulator Photodetectors”, 2nd International Conference on Group IV Photonics, FB 12005 (page 172, FIG. 3)    Non-Patent document 2: Tineke Thio, H. J. Lezec, T. W. Ebbesen, K. M. Pellerin, G. D. Lewen, A. Nahata, and R. A. Linke, “Giant optical transmission of sub-wavelength apertures: physics and applications”, Nanotechnology, vol. 13, pp. 429-432, FIG. 4.